Characterization of some metal-free organic dyes as photosensitizer for nanocrystalline ZnO-based dye sensitized solar cells

We report the photoelectrochemical characteristics of some biologically used dye: Bromophenol, Ponceau S, Sudan IV, Giemsa, and Acridine Orange as sensitizers. The J_SC from 2.5 to 0.47 mA cm⁻² with the order Bromophenol > Ponceau S > Sudan IV > Giemsa > Acridine orange, the V_OC from 642 to 384 mV, the fill factor (FF) from 0.61 to 0.40, and P_max from 855 to 84 μW cm⁻² were obtained from the DSSCs sensitized with these metal free organic dyes. Among these dyes, Bromophenol gave the best performance as sensitizer with maximum current, which is due to the better interaction between the hydroxyl groups of the dye on the surface of ZnO porous film. Incident photon- to-current conversion efficiency (IPCE) achieved with the use of these dyes follows the order Ponceau S > Sudan IV > Bromophenol > Giemsa > Acridine orange.     

An equivalent circuit approach to the modelling of the dynamics of dye sensitized solar cells

A model that can be used to interpret the response of a dye-sensitized photo electrode to intensity-modulated light (intensity modulated voltage spectroscopy, IMVS and intensity modulated photo-current spectroscopy, IMPS) is presented. The model is based on an equivalent circuit approach involving a transmission line with both an electrical and an ionic branch. An analytical expression including a term from the passive electrochemical impedance of the network, and a term accounting for the photo generation in the electrode is found. From this model IMVS and IMPS responses as well as iV curves can be calculated and used to optimize the photo electrode with respect to thickness and density. The result is mathematically equivalent to the usual approach for IMVS and IMPS modelling based on diffusion equations describing the transport of electrons in the semiconductor and on charge accumulation in traps, although these assumptions are not included in the transmission line model. The diffusion-like behaviour shows up as a consequence of the topology of the coupling between transport processes rather than as an inherent property of the electron transport itself. In this model electron trapping occurs because of electrostatic interactions between electrons in the semiconductor and ions in the electrolyte.     

A phenylcarbazole functionalized ruthenium dye for efficient dye-sensitized solar cells

A new heteroleptic Ru(II) complex of [Ru(Hcpip)(Hdcbpy)(NCS)₂]⁻·[N(C₄H₉)₄]⁺·H₂O {where Hcpip = 2-(4-(9H-carbazol-9-yl)phenyl)-1H-imidazo[4,5-f] [1,10]phenanthroline, Hdcbpy = 4-carboxylic acid-4′-carboxylate-2,2′-bipyridine} has been synthesized and demonstrated to function as an efficient sensitizer for nanocrystalline TiO₂-based dye-sensitized solar cell (DSSC). The DSSC based on this Ru(II) complex showed a short-circuit photocurrent density of 19.2 mA cm⁻², an open-circuit photovoltage of 630 mV, a fill factor of 57.7%, corresponding to an overall light to electricity conversion efficiency of 6.98% under simulated solar light irradiation at 100 mW cm⁻². This efficiency value is 2.81- and 1.08-fold efficiency values of 2.48% and 6.47% observed for carbazole-free parent complex [Ru(Hpip)(Hdcbpy)(NCS)₂]⁻·[N(C₄H₉)₄]⁺·H₂O {where Hpip = 2-phenyl-1H-imidazo[4,5-f][1,10]phenanthroline}- and cis-bis(isothiocyanato)bis(4,4′-dicarboxylic acid-2,2′-bipyridine)ruthenium(II) N3-based solar cells respectively, under identical experimental conditions. The molecular structures and electronic properties of the Ru(II) complexes were also investigated by means of density functional theory calculations in an effort to understand the device performance observed.     

The degradation of dye sensitized solar cell in the presence of water isotopes

The effects of water content on the degradation behavior of dye-sensitized solar cells were studied by adding water isotopes (H₂O, D₂O, and H₂¹⁸O) to the electrolytes. Time-dependent photovoltaic performance of the water-added DSSCs was monitored accompanied with the diffuse-reflectance infrared Fourier transform (DRIFT) technique and electrochemical impedance spectroscopy (EIS) measurement. DRIFT technique was utilized to study the chemical reactions that occurred on the working electrodes (TiO₂). EIS was implemented to evaluate the effects of the charge-transfer resistance at the interfaces between TiO₂/dye/electrolyte. Results show that the degradation rates of the cells in presence of water isotopes were in the order of H₂O>D₂O>H₂¹⁸O. In addition, the values of open-circuit voltage (V_oc) and fill factor (FF) for the water-added cells increased within first 12 h sun irradiation. However, their short-circuit current (J_sc) and efficiency (η) decreased during the sun irradiation. Moreover, a new peak assigned to LiNCS was noticed after soaking in water-added electrolytes for 48 h, attributing to the interaction of lithium ions with free thiocyanate ions from the dye.     

An improved preparation of 3-ethyl-1-methylimidazolium trifluoroacetate and its application in dye sensitized solar cells

In this paper, we reported an improved preparation of 3-ethyl-1-methylimidazolium trifluoroacetate (EMITA), which proceeded via efficient reaction of 1-methylimidazole and ethyl trifluoroacetate under solvent-free conditions using Teflon-lined, stainless steel autoclaves. It was shown that the procedure was simple and eco-friendly. The apparent diffusion coefficients of triiodide and iodide in binary ionic liquids, EMITA and 1-methyl-3-propylimidazolium iodide (MPII) with various weight ratios, were demonstrated by cyclic voltammetry using a Pt ultramicroelectrode. It was found that the apparent diffusion coefficients of triiodide slightly increased and those of iodide decreased with the weight ratio increase of EMITA and MPII. The dye sensitized solar cells with the electrolyte, which was composed of 0.13 M I₂, 0.10 M LiI, 0.50 M 4-tert-butylpyrdine in the binary ionic liquid electrolyte of EMITA and MPII (weight ratio 1:2), gave short circuit photocurrent density of 7.88 mA cm⁻², open circuit voltage of 0.61 V, and fill factor of 0.67, corresponding to the photoelectric conversion efficiency of 3.22% at the illumination (Air Mass 1.5, 100 mW cm⁻²).     

Environmental aspects of electricity generation from a nanocrystalline dye sensitized solar cell system

A Life Cycle Assessment, LCA, of a nanocrystalline dye sensitized solar cell (ncDSC) system has been performed, according to the ISO14040 standard. In brief, LCA is a tool to analyse the total environmental impact of a product or system from cradle to grave. Six different weighing methods were used to rank and select the significant environmental aspects to study further. The most significant environmental aspects according to the weighing methods are emission of sulphur dioxide and carbon dioxide. Carbon dioxide emission was selected as the environmental indicator depending on the growing attention on the global warming effect. In an environmental comparison of electricity generation from a ncDSC system and a natural gas/combined cycle power plant, the gas power plant would result in 450 g CO₂/kWh and the ncDSC system in between 19–47 g CO₂/kWh. The latter can be compared with 42 g CO₂/kWh, according to van Brummelen et al. “Life Cycle Assessment of Roof Integrated Solar Cell Systems, (Report: Department of Science, Technology and Society, Utrecht University, The Netherlands, 1994)” for another thin film solar cell system made of amorphous silicon. The most significant activity/component contributing to environmental impact over the life cycle of the ncDSC system is the process energy for producing the solar cell module. Secondly comes the components; glass substrate, frame and junction box. The main improvement from an environmental point of view of the current technology would be an increase in the conversion efficiency from solar radiation to electricity generation and still use low energy demanding production technologies. Also the amount of material in the solar cell system should be minimised and designed to maximise recycling.     

On the modeling of the dye-sensitized solar cell

A simplified electric model of the dye-sensitized electrochemical solar cell (DSC) is presented. It permits the calculation of internal steady-state cell characteristics like particle density distributions or the electric field as a function of the (measured) external current I_ext. The cell is modeled as an one-dimensional pseudo-homogeneous medium of thickness L, where all the electroactive particles involved in the current supporting process move according to different effective transport coefficients (i.e. effective diffusivities D and effective mobilities μ). The electroactive particles are the electrons e⁻ injected into the nanoporous TiO₂ layer after light absorption by the dye, the reduced and the oxidized counterpart of the redox electrolyte El_Red and El_Ox, and the positively charged cation Kat⁺ being brought into the cell together with the electrolyte. By applying the continuity equation, the transport-equation and Poisson's equation to all the electroactive species involved (e⁻, El_Red, El_Ox and Kat⁺) and by assuming a linear Boltzmann relaxation approximation for the back reaction, a system of differential equations is derived, describing particle densities, particle currents and the electric field within the cell. The underlying simplifying assumptions as well as the resulting limits of the model are stated, and some possible extensions are given. This paper aims to outline the general ideas and limitations of the proposed electric modeling, numerical calculations have been successfully implemented, but will be presented in a future paper.     

Study of hydrogen production in light assisted microbial electrolysis cell operated with dye sensitized solar cell

Hydrogen production with light as an additional energy source in a microbial electrolysis cell (MEC) is described. A ruthenium-dye (N719) sensitized solar cell with an open circuit potential (V_oc) of 602 mV was connected to the MEC. Hydrogen production was carried out by irradiating the DSSC connected across the MEC with a light intensity of 40 mW/cm² and also with natural sunlight. The DSSC was stable during various batch experiments. The acetate conversion efficiency and the coulombic efficiency based on the average of first two batches were 30.5 ± 2.5% and 40 ± 2% respectively. The cathodic recovery efficiency ranged from 72% to 86% during repeated batch experiments with an average of 78 ± 2.5%.     

Synthesis conditions, light intensity and temperature effect on the performance of ZnO nanorods-based dye sensitized solar cells

We present in this work a careful study of the different parameters affecting vertically-aligned ZnO-nanorods (NRs) based dye sensitized solar cells (DSCs). We analyze the effect of synthesis conditions, light intensity, UV light and working temperature, and correlated them to the final photovoltaic properties of the DSC. Although similar studies can be found in the literature for DSCs based on TiO₂, this work is, to our knowledge, the first detailed study carried out for DSC based on vertically-aligned ZnO nanorods. The ZnO NRs were grown between 1.6 and 5.2 μm long. Electrodes made with 1.6 ± 0.2 μm thickness were used to analyze parameters such as synthesis conditions, light intensity (800-1500 W m⁻²), UV light irradiation and temperature (25-75 °C). We have also carried out initial analysis of the solar cell lifetime under continuous light irradiation at 45 °C, and analyzed the ZnO electrode before and after testing. The best photovoltaic response was characterized by a power conversion efficiency of 1.02%, with J_sc of 3.72 mA cm⁻², V_oc of 0.603 V and 45% FF (at 72 °C), for a ZnO NR electrode of 5.2 μm thickness. Comparison of our power conversion efficiency values with published data is also presented, as well as a brief discussion on the possible reasons behind the low power conversion efficiency observed for these type of solar cells.     

Electrochemical reaction rates in a dye-sensitised solar cell—the iodide/tri-iodide redox system

The electrochemical reaction rate of the redox couple iodide/tri-iodide in acetonitrile is characterised by impedance spectroscopy. Different electrode materials relevant for the function of dye-sensitised solar cells (DSSC) are investigated. Preferably, the reaction with the iodide/tri-iodide couple should be fast at the counter electrode, i.e. this electrode must have a high catalytic activity towards the redox couple, and the same reaction must be slow on the photo electrode.The catalytic activity is investigated for platinum, poly(3,4-ethylenedioxythiophene) (PEDOT), polypyrrole (PPy), and polyaniline (PANI)—all deposited onto fluorine-doped tin oxide (FTO) glass. Both Pt and PEDOT are found to have sufficiently high catalytic activities for practical use as counter electrodes in DSSC. The reaction resistance on FTO and anatase confirmed the beneficial effect of a compact anatase layer on top of the FTO glass in lowering the tri-iodide reduction rate.     

Effect of a redox electrolyte in mixed solvents on the photovoltaic performance of a dye-sensitized solar cell

The effect of the iodide/triiodide redox electrolyte in various organic solvents on the photoelectrochemical properties of bis(tetrabutylammonium) cis-bis(thiocyanato)bis(4-carboxy-2,2′-bipyridine-4′-carboxylato)ruthenium(II)-sensitized nanocrystalline TiO₂ solar cells was studied. Solvents with large donor numbers dramatically enhanced the open-circuit voltage (V_oc), but usually reduced the short-circuit photocurrent density (J_sc). For a mixed solvent of tetrahydrofuran (THF) and acetonitrile, V_oc increased and the fill factor decreased with increasing THF concentration, but J_sc remained relatively constant. As the partial charge of the N or O atom of the solvent molecule increased, V_oc increased, but J_sc was unchanged up to a certain value of the partial charge (for THF, −0.46). For cells using 0.3 M 4-tert-butylpyridine and 20 vol% THF in the electrolyte, a short-circuit photocurrent density of 18.23 mA cm⁻², an open-circuit voltage of 0.73 V, a fill factor of 0.73, and an overall conversion efficiency of 9.74% were obtained.     

A novel approach to titania nanowire arrays as photoanodes of back-illuminated dye-sensitized solar cells

Titania nanowire arrays have been deposited on Ti foils through direct oxidizing the Ti substrate with aqueous hydrogen peroxide solutions containing melamine and nitric acid, and the applicability of such nanowire arrays to back-illuminated dye-sensitized solar cells studied in parallel with titania nanotube arrays on Ti foils. The low-temperature nitrogen adsorption measurement reveals that the film with nanowires 25 nm in diameter and 1 μm in length possesses a BET specific surface area of 59.0 m² g⁻¹, a value much larger than 26 m² g⁻¹ calculated for the nanotube with an inner diameter of 80 nm, an outer diameter of 120 nm and a total length of 3 μm. Assuming an indirect transition between band gaps, the nanowire film exhibits a bandgap of 3.1 eV, slightly larger than that of 3.0 eV for the nanotube one. A detailed electrochemical study suggests that, in comparison with the nanotube film, the nanowire one exhibits much lower saturated photocurrent and poorer conductivity under the Xe-lamp irradiation. However, when utilized to construct back-side illuminated dye-sensitized solar cells, the cell with the 2 μm-thick nanowire photoanode possesses significantly higher efficiency than the one with the 3 μm-thick nanotube photoanode. The relatively high energy conversion efficiency is contributed to the high specific surface area and the unique mesoporous structure of the titania nanowire arrays, which favors the adsorption of dye molecules.     

Preparation of 1-methyl-3-propylimidazolium acetate and its application in dye sensitized solar cells

In this paper, we reported the preparation of 1-methyl-3-propylimidazolium acetate (MPIAc), which proceeded via the metathesis of 1-methyl-3-propylimidazolium iodide (MPII) and lead acetate or potassium acetate. The apparent diffusion coefficients of triiodide and iodide in binary ionic liquids, MPIAc and MPII with various weight ratios, were demonstrated by cyclic voltammetry using a Pt ultramicroelectrode. It was found that the apparent diffusion coefficients of triiodide increased and those of iodide slightly increased with the weight ratio increase of MPIAc and MPII. The dye sensitized solar cells with the electrolyte, which was composed of 0.13 M I₂, 0.10 M LiI, 0.50 M 4-tert-butylpyrdine in the binary ionic liquid electrolyte of MPIAc (employing potassium acetate) and MPII (weight ratio 0.2), gave short circuit photocurrent density of 9.40 mA cm⁻², open circuit voltage of 0.62 V, and fill factor of 0.57, corresponding to the photoelectric conversion efficiency of 3.34% at the illumination (air mass 1.5, 100 mW cm⁻²).     

A quasi-solid-state dye-sensitized solar cell based on the stable polymer-grafted nanoparticle composite electrolyte

A novel gel electrolyte was prepared by dispersing the polymer-grafted ZnO nanoparticle into liquid electrolyte. This gel electrolyte behaves long-term stability as the poly(ethylene glycol methyl ether) molecules are strongly connected to ZnO nanoparticles with covalent bond in polymer-grafted ZnO nanoparticle. A quasi-solid-state dye-sensitized solar cell (DSC) based on this gel electrolyte yields the energy transfer efficiency of 3.1% at AM 1.5 direct irradiation of 75 mW cm⁻² light intensity. Addition of 4-tert-butylpyridine into the electrolyte results in dramatically improved short circuit current density I_sc, and the overall efficiency is also improved to 5.0%, while the open circuit voltage (V_oc) and fill factor (ff) are insensitive to the presence of 4-tert-butylpyridine. DSC fabricated with this novel gel electrolyte displays better thermal stability than those solidified with the conventional nanoparticle ZnO(Ac).     

Improved efficiency of dye sensitized solar cells by treatment of the dyed titania electrode with alkyl(trialkoxy)silanes

Dye sensitized cells are improved by passivation of the dyed titania electrode by silanizing the dyed surface with alkyl(trialkoxy)silanes. In cells utilizing the ruthenium dye bis(4,4′-dicarboxy-2,2′-bipyridine)bis(thiocyanato)ruthenium (II) (N3) optimum performance is produced by treating the dyed electrode with octyl(trimethoxy)silane in dry toluene. Such treatment increases efficiency as much as 66%, raising cells utilizing an ionic liquid electrolyte with high [I₃⁻] from 1.7% to 2.8% (1 sun AM1.5). The effect on dark currents and on cell efficiencies of this silanization and of dyeing both the FTO and TiO₂ surfaces is discussed for ionic liquid and acetonitrile based electrolytes.     

Influence of 1-methyl-3-propylimidazolium iodide on I3/I redox behavior and photovoltaic performance of dye-sensitized solar cells

In this paper, we investigated redox behavior of I⁻ and I₃⁻ in 3-methoxypropionitrile (MePN) with different concentrations of 1-methyl-3-propylimidazolium iodide (MPII) and iodine by cyclic voltammetry and electrochemical impedance spectroscopy. It was found that the apparent diffusion coefficient (D) values of triiodide and iodide ions, the serial resistance (R_s) and the charge-transfer resistance (R_ct) decreased slightly with increase of the concentration of I₃⁻ in MePN containing 1.4 M MPII. Moreover, the R_ct and D values of triiodide and iodide ions affection on dye-sensitized solar cells (DSCs) should be considered as a whole. The DSCs with the electrolyte (1.4 M MPII, 0.1 M LiI, 0.1 M I₂, 0.5 M TBP, in MePN) gave short circuit photocurrent density (J_sc) of 14.44 mA/cm², open circuit voltage (V_oc) of 0.72 V, and fill factor (FF) of 0.69, corresponding to the photoelectric conversion efficiency (η) of 7.17% under one Sun (AM1.5).     

Influence of alkylpyridine additives in electrolyte solution on the performance of dye-sensitized solar cell

The influence of alkylpyridines additive to an I⁻/I₃⁻ redox electrolyte in acetonitrile on the performance of a bis(tetrabutylammonium)cis-bis(thiocyanato)bis(2,2′-bipyridine-4-carboxylic acid, 4′-carboxylate)ruthenium(II) dye-sensitized TiO₂ solar cell was studied. I–V measurements were performed using more than 30 different alkylpyridines. The alkylpyridine additives showed a significant influence on the performance of the cell. All the additives decreased the short-circuit photocurrent (J_sc), but most of the alkylpyridines increased the open-circuit photovoltage (V_oc) and fill factor (ff) of the solar cell. The results of the molecular orbital calculations suggest that the dipole moment of the alkylpyridine molecules correlate with the J_sc of the cell. These results also suggest that both the size and ionization energy of pyridines correlate with the V_oc of the cell. Under AM 1.5 (100 mW/cm²), the highest solar energy conversion efficiency (η) of 7.6% was achieved by using 2-propylpyridine as an additive, which was more effective than the previously reported additive, 4-t-butylpyridine.     

Patterns of efficiency and degradation in dye sensitization solar cells measured with imaging techniques

A larger number of dye sensitization solar cells based on cis-Ru^II(LH₂)₂(NCS)₂ with LH₂=2,2′-bipyridyl-4,4′-dicarboxylic acid with an electrolyte consisting of 0.5 M LiI, 50 mM I₂, 0.2 M tert.-butyl pyridine in acetonitrile have been studied, using spatially resolved photocurrent imaging techniques. Measurements have been made after preparation and periodically during a longer period of simulated solar light illumination. The observed phenomena have been grouped into five categories. The first one concerns significant inhomogeneities reflecting the TiO₂-layer preparation technique used. The second category concerns an inhomogeneous deterioration of the dye sensitization cell during illumination. The third phenomenon involves photodegradation itself, which can be visualized by selectively illuminating the dye sensitization solar cell. Changes observed in the composition of the electrolyte, typically indicated by a bleaching of the iodide/iodine solution were also observed. Finally, the fifth category to be considered deals with a loss of electrolyte and the parallel appearance of gas bubbles in the solar cell. All these phenomena may coexist, being responsible for the overall process of degradation. The different mechanisms are discussed and analyzed in an effort to determine parameters critical for increasing efficiency and stability of dye sensitization solar cells.     

Method for fabricating the compact layer in dye-sensitized solar cells by titanium sputter deposition and acid-treatments

Dye-sensitized solar cells (DSCs) have been put forward as a potential low-cost alternative to the widely used silicon solar cells, which are subject to cost limitations. However, some problems need to be solved in order to enhance the efficiency of DSCs. In particular, the electron recombination occurred by the contact between the transparent conductive oxide (TCO) and a redox electrolyte is one of the main limiting factors of efficiency. Accordingly, a compact layer plays an important role in realizing highly efficient DSCs because it improves the adhesion of the TiO₂ to the TCO and provides a larger contact area and more effective electron transfer by preventing electron recombination. In this work, the fabrication of a TiO₂ compact layer using Ti sputter deposition and acid-treatment was investigated rather than the conventional method, which uses a TiCl₄ aqueous solution. The acid-treatment of the sputtered Ti film actively oxidized the Ti particles. As a result, such a cell exhibited an additional 1.3% in total efficiency compared to the standard DSC without a compact layer. These improvements are not inferior to those obtained by the conventional fabrication method using a TiCl₄ aqueous solution.     

Influence of pyrazole derivatives in I/I3 redox electrolyte solution on Ru(II)-dye-sensitized TiO2 solar cell performance

The influence of pyrazole additives in an I⁻/I₃⁻ redox electrolyte solution on the performance of a bis(tetrabutylammonium)cis-bis(thiocyanato)bis(2,2′-bipyridine-4-carboxylic acid, 4′-carboxylate)ruthenium(II) (N719) dye-sensitized TiO₂ solar cell was studied. The current–voltage characteristics of the cell were measured using 18 different pyrazole derivatives. All of the pyrazole additives enhanced the open-circuit photovoltage (V_oc) and the solar energy conversion efficiency (η), but reduced the short-circuit photocurrent density (J_sc). Most of the pyrazoles improved fill factor (ff). The physical and chemical properties of the pyrazoles were computationally calculated in order to elucidate the reasons for the additive effects on cell performance. The greater the partial charge of the nitrogen atom at position 2 in the pyrazole group, the larger the V_oc, but the smaller the J_sc values. As the dipole moment of the pyrazole derivatives increased, the V_oc value increased, but the J_sc value decreased. The V_oc of the cell also increased as the ionization energy of the pyrazoles decreased. These results suggest that the electron donicity of the pyrazole additives affected the interaction with the nanocrystalline TiO₂ photoelectrode, the I⁻/I₃⁻ electrolyte, and the acetonitrile solvent, which changed the Ru(II)-dye-sensitized solar cell performance.